1. Field of the Invention
The present invention relates to an oxygen detector in which a change in the amount of oxygen in an atmosphere is made visible by a change in color, and a method for manufacturing an oxygen detector.
2. Description of the Related Art
In the storage of food, medicine, or the like, oxygen in the atmosphere oxidizes the food, the medicine, or the like and decreases the quality of the food, the medicine, or the like. Then, in order to prevent the decrease in quality during storage, the food, the medicine, or the like is placed, together with a deoxidizer, in a packaging container (including a packaging bag) and hermetically packaged. Oxygen in the packaging container can be absorbed by the deoxidizer or the like, and the food, the medicine, or the like can be stored in an oxygen-free state (for example, an oxygen concentration of 0.1% or less).
In recent years, an oxygen detector has been enclosed in a packaging container, together with a deoxidizer. The oxygen detector has been used to detect the presence or absence of oxygen in the packaging container. The presence or absence of oxygen in a hermetic packaging container is made visible by a change in color of the oxygen detector. The user can easily check whether food, medicine, or the like is stored in an oxygen-free state or not, based on the color of the oxygen detector.
This type of oxygen detector generally comprises a reducing saccharide, a basic substance, and a redox dye which has the ability of change color depending on its oxidative state (for example, Japanese Patent: Japanese Patent No. 2580157 B2, and Japanese Patent Application: Japanese Patent Laid-Open No. 2007-3259 A1). The reducing saccharide is used to maintain the redox dye in the reduced state when the atmosphere is in an oxygen-free state. When the redox dye maintained in the reduced state is oxidized by oxygen in the atmosphere, the redox dye changes the color. In this manner, the oxygen detector detects oxygen, using a mechanism of changing color of the redox dye depends on its oxidative state. Therefore, a clear color change and a quick color change response accompanying a change in the amount of oxygen in the atmosphere are required of the oxygen detector. Then, in the above Patent Documents, a change in color caused by the oxygen detector between the reduced state and the oxidized state is made clearer, using a dye not reduced by the reducing saccharide, such as a red food coloring.
The oxygen detector is manufactured by preparing an oxygen indicator aqueous solution (oxygen detecting solution) for an oxygen detector comprising a reducing saccharide, a basic substance, and a redox dye and allowing this oxygen indicator aqueous solution to be carried on various carriers, such as a sheet-shaped carrier and a tablet. The reducing saccharide is ring-opened in the oxygen indicator aqueous solution adjusted to be basic by the basic substance and forms a chain structure having a reducing group (an aldehyde group or a ketone group), which reduces the redox dye. Conventionally, monosaccharides having large reducing power, such as D-glucose, have been mainly used as such a reducing saccharide.
However, conventional problems are that the oxygen detector has low heat resistance. That is, when the oxygen detector is stored at high temperature (for example, a temperature equal to or higher than room temperature, 35° C.), the ability to detect oxygen decreases. Therefore, a quick color change response is not obtained and a color tone change is unclear when the amount of oxygen in the atmosphere changes.
A cause of the decrease in the ability to detect oxygen during storage at high temperature is the browning of the reducing saccharide. The browning of the reducing saccharide is caused by the fact that the reducing saccharide forming the chain structure in the aqueous solution reacts with the basic substance and is decomposed from an end having the reducing group. This browning of the reducing saccharide tends to occur as the atmospheric temperature increases. Particularly, monosaccharides have large reducing power and high reactivity, and therefore, browning tends to occur. When the reducing saccharide browns in the oxygen indicator aqueous solution, the color of the oxygen detector also turns brown, and therefore, a color change is unclear. In addition, when the reducing groups are consumed due to the progress of the reaction with the basic substance, it is difficult to maintain the redox dye in the reduced state in an oxygen-free state, and part of the redox dye may have an oxidized structure. In this case, the ability to detect oxygen decreases, and even if oxygen is detected, a color change may be unclear, and a quick color change response may not be obtained. Therefore, conventionally, by storing the oxygen detector at low temperature (for example, 10° C. or less) before shipment, the browning of the reducing saccharide has been prevented to prevent degradation in the ability of the oxygen detector to detect oxygen.
Then, it is an object of the present invention to provide an oxygen detector that has high heat resistance, can be stored at room temperature, and can maintain an excellent ability to detect oxygen, regardless of the atmospheric temperature, and a method for manufacturing an oxygen detector.